How to Make It
Fit a steamer basket inside a large pot. Fill pot with water until water line is just below bottom of steamer basket. Bring to a rapid boil over high. Place crabs in steamer basket. Cover pot, and reduce heat to medium-high; steam crabs 18 minutes. Meanwhile, fill a large bowl with ice water.
Remove steamed crabs from pot, and plunge into ice water; let stand 10 minutes. Working with 1 crab at a time, remove from ice water. Remove crabmeat from shells; transfer crabmeat to a bowl, and discard crab shells. (Crabmeat should yield about 1 pound.)
Preheat oven to 350°F. Melt 1/2 cup butter in a medium skillet over medium. Add scallions and celery leaves; cook, stirring occasionally, until tender, 3 to 5 minutes. Using a slotted spoon, remove mixture from skillet; transfer to a plate. Add lemon juice, 1/2 teaspoon salt, white pepper, and cayenne pepper to skillet. Bring to a simmer over medium-high; cook, undisturbed, until slightly reduced, about 2 minutes. Add crabmeat; cook, stirring gently, until warmed through, about 2 minutes. Remove from heat. Transfer crabmeat mixture to 4 (12-ounce) baking dishes or 1 (2-quart) baking dish; sprinkle with scallion mixture.
Melt remaining 1 tablespoon butter in skillet over medium. Add breadcrumbs and remaining 1/4 teaspoon salt; cook, stirring often, until breadcrumbs are well coated with butter and slightly browned, 4 to 5 minutes. Sprinkle breadcrumbs over crabmeat mixture.
Transfer baking dishes to preheated oven; bake until crab casseroles just begin to bubble around edges, about 20 minutes. Serve hot with tossed salad.